The invention relates to a system for transmitting alarm signals comprising at least one portable transmitting unit and one central receiver, the transmitting unit comprising a control circuit which is connected to an activation device and a transmitting circuit, the control circuit activating the transmitting circuit when the activation device is energized in order to transmit an alarm signal and in order to re-activate the transmitting circuit for the re-transmission of the alarm signal after the elapse of a delay time, the delay time being different from a delay time of another transmitting unit, the transmitting unit having a memory for storing an identification code allocated to the transmitting unit, the central receiver comprising a processing system which is connected to a receiving circuit which is suitable for receiving an alarm signal and the processing circuit activating a signalling device, on receipt of an alarm signal, for the purpose of generating an attention signal.
A system of this type is disclosed by the U.S. Pat. No. 4,347,501. In the known system, the activation device is a switch to be operated by a user of the transmitting unit. The delay times of all the transmitting units are different. This is intended to reduce the possibility that alarm signals transmitted by different transmitting units overlap one another and thereby interfere with their reception in the central receiver if the users of said transmitting units operate the switch of their transmitting unit approximately at the same instant. Operation of the switch approximately simultaneously may occur, for example, in an alert situation which is observed by different users of such transmitting units. In this connection, depending on the use of the system, a very serious situation may arise if none of the alarm signals transmitted approximately simultaneously is received well by the central receiver.
The known system has the drawback that a large number of different delay times, which therefore comprise a number of relatively long delay times, has to be used, as a result of which it may be a relatively long time before a transmission of an alarm signal is repeated, and this is very undesirable in some alert situations.